1. The Field of the Invention
The present invention relates generally to apparatus and methods for the in-situ formation of structural prostheses and particularly for in-situ formation of structural prostheses for the spine.
2. The Relevant Technology
Depicted in FIG. 1 is a section of a spinal column 10. Spinal column 10 comprises a plurality of stacked vertebrae 12. In general, each vertebrae 12 iscomprised of a substantially cylindrical vertebral body 14 having a spinous process 16 projecting therefrom. Spinal column 10 further comprises an intervertebral disc 18 located between each adjacent vertebrae 12. As depicted in FIG. 2, intervertebral disc 18 generally consists of an outer ring structure called the annulus fibrosus 20. Annulus Fibrosus 20 encircles a gelatinous central core called the nucleus pulposus 22.Intervertebral disc 18 is comprised of collagen with annulus fibrosus 20 being significantly stiffer than the gelatinous nucleus pulposus 22. In this regard, annulus fibrosus 20 functions in part as a wall that retains nucleus pulposus 22. Intervertebral disc 18 together with the two adjacent vertebrae 12 form a joint motion segment that serves to provide limited motion in forward bending, lateral bending, and rotation.
Degenerative discs 18 can cause debilitating back pain. Discs 18 progressively degenerate during aging, characterized by dehydration and hardening of the nucleus pulposus 22 and the annulus fibrosus 20. The annulus fibrosus 20 may weaken and bulge, or may develop fissures that allow the nucleus pulposus 22 to extrude, commonly referred to as disc herniation. This bulging or extrusion often results in a decrease in disc height, thereby putting pressure on nerve roots and/or the spinal cord.
Various prefabricated prostheses have been developed to repair or replace a damaged intervertebral disc 18, including: prostheses for the replacement of the nucleus pulposus, commonly referred to as nucleus replacements; prostheses for the concurrent replacement of the annulus fibrosis, commonly referred to as a total disc replacement; and prostheses in the form of cages filled with osteogenic materials, commonly referred to as interbody fusion devices. These prefabricated prostheses are commercially offered in a limited number of sizes, limiting the surgeon's ability to precisely restore the disc height for individual patients. Furthermore, most of these prostheses require the creation of a surgical incision at least large enough to pass the implant to the site of repair. Surgical incisions cause disruptions and damage to various skin, muscle, tendon and ligament structures that extend the time of recover and rehabilitation for patients and that compromise the function of the violated anatomical structures.
More recently, disc replacements have been developed that use flowable biomaterials that harden in-situ to form a replacement nucleus pulposus. Forming a prosthesis in-situ from a flowable biomaterial potentially facilitates a minimally invasive approach (i.e., no resection of tissue) to the repair site, thereby minimizing the damage to anatomical structures and enabling much quicker patient recoveries to full function. However, nucleus replacements formed from flowable biomaterials either require an implantable mold, such as that disclosed by U.S. Pat. Nos. 3,875,595 to Froning and U.S. Pat. No. 5,549,679 to Kuslich and U.S. Patent Application Publication No. 2001/004710 to Felt et al., or utilize the existing annulus fibrosus as the mold as disclosed in U.S. Patent Application Publication No.'s U.S. 2002/0049498 to Yussel et al. and U.S. 2002/0045942 to Ham, and U.S. Pat. No. 6,183,518 to Ross et al.
Several disadvantages exist with both the implantable mold and the use of the existing annulus fibrosus as the mold. An implantable mold cavity creates an additional interface between the mold and the flowable biomaterial which may be subject to interfacial shear stresses producing interfacial motions that create wear debris and compromise the structural integrity of both the mold and core formed from the flowable biomaterial. Using the existing annulus fibrosus as the mold is also problematic in that the annulus fibrosus is often degenerated so as to have one or more fissures extending therethrough. Such fissures allow the injected flowable biomaterial to escape.